I want track a site's visitor retention using Google Analytics, to see if unregistered users are returning to it, within a time frame of 2+ months from now.

This blog post seems to be on the right track, but I want to track unregistered users, so I don't have a "join date" or similar variable at my disposal.

This other blog post suggests using all 5 GA custom variables, using the first variable slot on the first week, variable 2 on week 2, etc. This method will allow me to track 5 weeks of visitors.

I want to track more than 5 weeks of visitors, so I was thinking on using two custom variables in GA: visitor's first visit date, and visitor's last visit date.

How I can save the first visit date? Because if I save another value in the same slot the old value will be overwritten, and I don't know how reliable is to save that variable conditionally (reading the __utmv cookie to check whether the "first visit date" is set, if it isn't set I save the current date)